U.S. patent number 6,522,437 [Application Number 09/784,608] was granted by the patent office on 2003-02-18 for agile multi-beam free-space optical communication apparatus.
This patent grant is currently assigned to Harris Corporation. Invention is credited to Michael O'Reilly, Harry Presley.
United States Patent |
6,522,437 |
Presley , et al. |
February 18, 2003 |
**Please see images for:
( Certificate of Correction ) ** |
Agile multi-beam free-space optical communication apparatus
Abstract
An electronically agile multi-beam optical transceiver has a
first crossbar switch, that switches input signals to selected ones
of a spatial array of light emitters. The light emitters supply
modulated light beams to spatial locations of a telecentric lens,
which geometrically transforms the beams along different divergence
paths, in accordance with displacements from the lens axis of the
light emitter elements within the spatial array. Light beams from
remote sites incident on a divergence face of the telecentric lens
are deflected to a photodetector array, outputs of which are
coupled to a second crossbar switch. An auxiliary photodetector
array monitors optical beams from one or more sites whose spatial
locations are known, and supplies spatial error correction signals
for real-time pointing and tracking and atmospheric correction.
Inventors: |
Presley; Harry (Malabar,
FL), O'Reilly; Michael (Melbourne, FL) |
Assignee: |
Harris Corporation (Melbourne,
FL)
|
Family
ID: |
25132981 |
Appl.
No.: |
09/784,608 |
Filed: |
February 15, 2001 |
Current U.S.
Class: |
398/128;
398/139 |
Current CPC
Class: |
H04B
10/1125 (20130101) |
Current International
Class: |
H04Q
11/00 (20060101); H04B 010/00 (); H04B
010/10 () |
Field of
Search: |
;359/663,662,619-626,131,152,128,159,19,27 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
E Brookner, "Atmospheric Propagation and Communication Channel
Model for Laser Wavelengths", IEEE Transactions On Communication
Technology, vol. COM-18, No. 4, Aug. 1970, pp 396-418..
|
Primary Examiner: Chan; Jason
Assistant Examiner: Bello; Agustin
Attorney, Agent or Firm: Allen, Dyer, Doppelt, Milbrath
& Gilchrist, P.A.
Claims
What is claimed is:
1. An electronically agile multi-beam optical transceiver
comprising a first crossbar switch, having inputs thereof adapted
to receive digital input communication signals, and outputs thereof
selectively coupled to light emitter elements of a two-dimensional
spatial array, that are operative to provide output beams conveying
said digital input communication signals to a plurality of spatial
locations of a telecentric lens system, said telecentric lens
system being configured to perform a geometric transform of a
respective one of said output beams, from a spatial location of
said telecentric lens system, along a divergence path passing
through a focal point lying on a lens axis in a lens aperture in
accordance with spatial displacement from said lens axis of an
associated light emitter within said two-dimensional spatial array,
and wherein said telecentric lens system is configured to allow
light beams at a transmission wavelength generated by said light
emitter elements to pass to and diverge from a light beam diverging
face of said telecentric lens system, and to deflect light incident
at a receiver wavelength upon said light beam diverging face of
said telecentric lens system to a photodetector array, outputs of
which are coupled to a second crossbar switch from which digital
output communication signals conveyed by light beams incident upon
said light beam diverging face of said telecentric lens system at
said receiver wavelength are derived.
2. An electronically agile multi-beam optical transceiver according
to claim 1, further including an auxiliary array of photodetector
elements arranged to monitor one or more optical beams from one or
more sites whose spatial locations are known, and being operative
to supply spatial error correction signals for controlling said
first and second crossbar switches so as to provide for real-time
pointing/tracking and atmospheric correction capability.
3. A method of performing point-to-multi-point communications for a
plurality of first communication signals from a first communication
site to a plurality of spatially diverse second communication
sites, comprising the steps of: (a) modulating respective ones of a
plurality of optical beams with said first communication signals;
and (b) selectively coupling said plurality of first optical beams
to a plurality of spatial locations of a telecentric lens system,
said telecentric lens system being configured to perform a
geometric transform of a respective one of said first optical
beams, from its spatial location of said telecentric lens system,
along a beam divergence path passing through a focal point lying on
a lens axis in a lens aperture, that diverges from said lens axis
in accordance with said spatial displacement from said lens axis of
said spatial location, and wherein step (b) comprises controllably
coupling said first communication signals by way of a first
crossbar switch to respective light emitter elements, said light
emitter elements being operative to provide said first optical
output beams conveying said first communication signals to selected
spatial locations of said telecentric lens system, and wherein said
telecentric lens system is configured to allow said first optical
output beams at a transmission wavelength generated by said light
emitter elements to pass to and diverge from a light beam diverging
face of said telecentric lens system, and to deflect light incident
at a receiver wavelength upon said light beam diverging face of
said telecentric lens system to a photodetector array, outputs of
which are coupled to a second crossbar switch from which digital
output communication signals conveyed by light beams incident upon
said light beam diverging face of said telecentric lens system at
said receiver wavelength are derived.
4. A method according to claim 3, further including the steps of:
(c) monitoring by way of an auxiliary array of photodetector
elements one or more optical beams from one or more sites whose
spatial locations are known; and (d) supplying spatial error
correction signals for controlling said first and second crossbar
switches in accordance with outputs of said auxiliary array of
photodetector elements.
Description
FIELD OF THE INVENTION
The present invention relates in general to optical communication
systems, and is particularly directed to a new and improved,
electronically agile, free-space optical communication apparatus,
that is configured to provide for selectively directing each of a
plurality of independent optical beams, such as those modulated
with respectively different communication signals, through a common
optical aperture in respectively different directions to a
plurality of spatially diverse receiver sites.
BACKGROUND OF THE INVENTION
Currently available optical (e.g., laser-based) communication
systems intended for free space applications, such as
building-to-building local area networks and trunk extension
systems, are customarily configured as (short range and long range)
free space `point-to-point` systems. As shown diagrammatically in
FIGS. 1 and 2, such systems typically include local and remote
optical (laser-based) transceiver pairs 1/2 and 4/5, each of which
has an associated telescope for an aperture, and are optically
coupled to one another over one or more line-of-sight optical links
3/6.
As further shown in FIG. 2, for long range applications in excess
of a few km, some form of actively driven mechanical stabilization
platform 7 is customarily used to maintain beam pointing. In
addition, for point-to-point applications that are consistent with
hub-spoke operation, the systems have a highly integrated
configuration, such as that shown in FIG. 3, and require a
substantial amount of hardware to provide multiple point-to-point
links between a high power hub site 8 and a plurality of receiver
(subscriber) sites 9. Unfortunately, none of these existing
architectures addresses tactical applications or mobile nodes, nor
do they provide for low cost point-to-multipoint
communications.
SUMMARY OF THE INVENTION
In accordance with the present invention, advantage is taken of
recent and emerging technology developments in free-space optical
communications (FSOC), including economically produced dense arrays
of addressable transmitter and receiver elements, to provide an
lectronically agile multi-beam optical transceiver (or `AMOX`) for
use in a point-to-multipoint hub, that allows any of multiple
optical beams (independently modulated with respectively different
communication signals), to be dynamically routed and spatially
re-directed, as desired, in respectively different directions
through a common optical aperture over a relatively wide field to a
plurality of spatially diverse sites or nodes. The invention also
includes a tracking array that actively corrects for pointing and
tracking errors that may be due to relative node motions and
atmospheric induced distortions. Being electronically agile, the
invention has no moving parts, and thus achieves a reduction in
size, weight, and cost, while improving reliability and
functionality.
To this end, a multiport input-output unit contains an input
crossbar switch, respective inputs of which are supplied with
electronic signals, such as subscriber signals supplied by way of a
digital telecommunication network. The crossbar switch's outputs
are connected to respective transmitter driver circuits coupled to
a (two-dimensional) array of light emitter (laser) elements, whose
output beams are coupled to a telecentric lens system. For an
integrated transceiver application, the telecentric lens system
contains a frequency-selective (dichroic) interface that allows
light at the transmission wavelength generated by the light emitter
array to pass to and diverge from a convex face of the lens,
whereas light incident upon the lens's convex face is reflected by
the dichroic interface to an opto-electronic receiver array.
The telecentric lens performs a geometric transform of a beam from
a spatial location of the transmit array along a path passing
through a focal point within an aperture at the exit face of the
lens diverges in accordance with the two-dimensional spatial
displacement from the beam axis of its associated emitter within
the transmitter array. This means that the desired travel path of
an optical beam carrying a particular signal channel may be readily
defined by controlling the crossbar switch feeding the
two-dimensional transmitter array. Thus, the invention is able to
project multiple transmit optical signals from a two-dimensional
planar array of optical emitters into differentially divergent,
free-space beams through a commonly shared aperture of the
telecentric lens, with a precise relationship between the position
of an emitter and it's angular transmit direction.
In the receive or return path direction, the telecentric lens
accepts multiple receive optical beams and directs them onto a
two-dimensional receiver array. The optics of the lens system
produce a typical Fourier transform operation, and the focal plane
positions correspond to unique angular beam directions. The
photodetector array has its outputs connected to respective signal
demodulators outputs of which are coupled to an receiver side
crossbar switch, outputs of which are supplied to digital
subscriber lines coupled to the transmit crossbar switch.
An auxiliary tracking (two-dimensional) photodetector array may be
used to monitor one or more beams from nodes whose spatial
locations relative to the hub site are precisely known. Any offset
in the spatial location of a `tracking` beam from such a node on
the tracking array is used as an error correction signal by the
control processor to impart the appropriate (X-Y) correction, as
needed, in the steering commands supplied to the crossbar switches
so as to provide for real-time pointing/tracking and atmospheric
correction capability.
In some applications, the transmit and receive beams may be split
between two spatially separate apertures, so that (transmit vs.
receive) wavelength segregation is not necessary. Potential
advantages of such beam division include larger receiver apertures
for improved signal collection, optimization to specific transmit
and receive array configurations, and a reduction in the complexity
of diffractive optical elements or holographic optical
elements.
The transmitter array may be implemented in a variety of ways.
Where the number of remote nodes, which are generally spatially
stable, is small, a sub-populated non-switchable or `non-agile`
array may be employed. An example of a `non-agile` application
involves the use of an Ethernet network to `locally` connect
buildings that are reasonably close to one another. A limited set
of discrete laser sources may be hard-wired via an array of
associated optical fibers to respective spatial locations within a
light emitter array plane, for which the spatial-to-angular
transform produced by the telecentric lens will direct the emitter
beams along angular directions to subscriber nodes.
Although the invention may be applied to such `non-agile`
multi-beam terminals, the preferred embodiment of the invention
employs the `agile` configuration described above, in which any
array position is potentially active and dynamically addressable. A
non-limiting application of an agile array would be to allow mobile
communication personnel to rapidly deploy a local area network
(LAN), while providing for dynamic variations in the number and/or
physical locations of the nodes of the network, and to track and
correct for relative motion between the nodes.
To realize cost-effective, agile transmitter arrays, vertical
cavity surface-emitting laser (VCSEL) components may be employed in
combination with an M.times.N digital crossbar switch.
Alternatively, the VCSELs may be replaced by discrete laser diodes
in a sub-populated array. An advantage of VCSELs is their ability
to simultaneously emit multiple transverse modes (MTMs). A
multi-transverse mode source may reduce the effects of atmospheric
scintillation in a FSOC link. With an MTM source, the beam is
already somewhat homogenized, so that additional phase scrambling
due to scintillation may be greatly reduced. This effect may also
be generated or enhanced by using a custom-designed optical element
to scramble the phase-fronts prior to transmission. As a
non-limiting example, a DOE/HOE or a simple diffuser may be
employed. This technique may also be used to produce the desired
beam angle for the intended application.
As an alternative to electronic configurations, each crossbar
switch may be implemented as an all-optical fiber optic switch. A
principal advantage of an optical fiber approach is that the number
of laser elements can be reduced to match the number of input
signals. The transmit element array may comprise a fully populated
fiber optic bundle, which can be configured and sized to have the
desired element center-to-center spacing.
Although the transmitter array may comprise a spatially periodic,
two-dimensional array of point-source emitters, the beams impinging
upon the receiver array can be expected to be incident at arbitrary
locations within the array depending on the angular position of
subscriber nodes. The receiver array elements should therefore have
the largest possible active area (up to the desired spatial
resolution of the array) and the highest possible fill-factor (or
very little dead space between photodetector elements). Also, the
node connecting the detector, preamplifier, and feedback resistor
components of a respective photodetector element must be relatively
`physically short` in order to preserve the receiver's bandwidth
performance. In a two-dimensional receiver array, this node length
may become unacceptable due to the loss of the second dimension for
mounting components. The receiver array may be configured as a
fiber bundle outputs of which are (optical-fiber) routed via a set
of fiber optic switches to a subset of optimized discrete
photodetectors.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 diagrammatically illustrates a conventional short range,
free space optical laser-based) communication system;
FIG. 2 diagrammatically illustrates a conventional long range, free
space optical laser-based) communication system;
FIG. 3 diagrammatically illustrates hub/spoke-configured multiple
point-to-point free space optical communication system;
FIG. 4 diagrammatically illustrates an (electronically) agile
multi-beam optical transceiver in accordance with the
invention;
FIG. 5 shows an example of a telecentric lens configuration that
may be used in the transceiver of FIG. 4;
FIG. 6 depicts a telecentric lens configuration for a
unidirectional terminal;
FIG. 7 is a beam-forming geometry diagram associated with a common
aperture;
FIG. 8 diagrammatically illustrates a non-agile multi-beam optical
transmitter for electrical input signals;
FIG. 9 diagrammatically illustrates a non-agile multi-beam optical
transmitter for fiber optic input signals;
FIG. 10 shows an electronically agile transmitter array employing
vertical cavity surface-emitting lasers coupled with a crossbar
switch;
FIG. 11 shows an electronically agile transmitter array employing
discrete laser diodes in a sub-populated array coupled with a
crossbar switch;
FIGS. 12 and 13 show respective transmitter arrays employing a
fiber optic crossbar switch;
FIG. 14 shows an example of compiled results of link analyses for
determining array size and addressable field-of-regard; and
FIG. 15 shows an alternative embodiment of a receiver array.
DETAILED DESCRIPTION
A non-limiting embodiment of the multi-beam communication apparatus
in accordance with the present invention, configured as an
(electronically) agile multi-beam optical transceiver (AMOX) for
use in a point-to-multipoint hub, is diagrammatically illustrated
in FIG. 4 as comprising a multiport input-output unit shown in
broken lines 10, that is coupled to receive electronic signals,
such as those provided by way of a variety of signal transport
paths, including (subscriber) signals supplied by way of a digital
telecommunication network. As a non-limiting example, in the
transmit direction, the input-output unit 10 may include an M input
by N output crossbar switch 11 of the type typically installed as
part of a telecommunication service provider's central office
equipment.
The crossbar switch 11 serves to enable a signal applied to any
input port of an array of M input ports 12 to be controllably
electronically steered (by an associated control processor 100) to
any output port of an array of N output ports 13. The N output
ports 13 of the switch 11 are connected, in turn, to respective
signal inputs 21 of a set of transmitter driver circuits 20,
outputs 22 of which are coupled to signal inputs of an integrated
array of light emitter elements 30. While the light emitter array
30 may comprise a 1.times.K array of elements, in a preferred
embodiment for expanded volume multipoint transmission coverage,
the light emitter array 30 is configured as a two-dimensional
spatially array of light emitting elements (e.g., lasers), output
beams of which have a prescribed optical transmission wavelength
.lambda..sub.T. As a non-limiting example, array 30 may comprise a
laser emitter array available from Novalux Inc., Sunnyvale, Calif.,
having a substantially planar output surface 32, which facilitates
intimately optically coupling the array with a substantially planar
input face 41 of a telecentric lens system 40.
The telecentric lens system 40 may comprise a first lens element 50
having a first, substantially planar face 51 and a second, convex
face 52 that is optically coupled with an adjoining second,
convex-convex optical beam translating lens element 60. For the
case of a two-dimensional light emitter array, the geometrical
surfaces of the lens elements of the telecentric lens, that are
intersected by an axis 70 orthogonal to the center of the laser
element array 30, are surfaces of revolution, symmetric about the
axis 70.
For the present transceiver example, the lens element 50 may be
formed by bonding first and second lens block components 53 and 54
to a frequency-selective (dichroic) interface 55, that allows light
at the transmission wavelength .lambda..sub.T generated by the
light emitter element array 30 to pass through the interface 55 and
exit the second, convex face 52, whereas light having a different
receiver wavelength .lambda..sub.R as received by the face 52 from
the lens element 60 is reflected by the lens' dichroic interface 55
towards a side face 56, to which an opto-electronic receiver array
130 is coupled. The receiver array preferably includes a front end
normal-incidence bandpass filter. This filter, in conjunction with
the wavelength selective dichroic mirror in the telecentric lens
arrangement, is effective to efficiently filter background light
from the received signals.
In an alternative configuration, the lens element 50 may be
implemented as two sub-components, as shown at 80 and 90 in FIG. 5
(which illustrate transmit and receive beams associated with three
duplex channels). In the telecentric lens configuration of FIG. 5,
the first sub-lens component 80 is formed of two bonded components
with a dichroic interface 55 therebetween, as in the architecture
of FIG. 4. The sub-lens component 80 has a first substantially
planar face 81 to which the light emitter array 30 is coupled, and
a second planar face 82 that adjoins an associated planar face 91
of the second sub-lens component 90. The sub-lens component 90 has
a convex face 92 that is optically coupled with the adjoining
convex-convex optical beam translating lens element 60.
As shown in FIG. 4, the telecentric lens arrangement 40 is
effective to perform a geometric transform of an optical beam
incident upon the generally planar surface 51 of the lens element
50, along a path passing through and diverging from a focal point
62 within an aperture 64 at the exit face 66 of the lens element
60. As shown in FIG. 5, the parameters of the lens system are such
that the diameter of the aperture 64 is sufficient to accommodate
spreading of each of the transmit beams from its respective emitter
within the array 30. The transmit beams (having transmission
wavelength .lambda..sub.T) are de-focused to the desired amount of
angular beam width by simply controlling the distance between the
surface 51 to which the array 30 is coupled and the telecentric
lens. This does not impact the steering direction of the beams.
The geometric transform performed by the telecentric lens is such
that the angle .alpha. subtended by the travel path of a beam
exiting the exit face 66 of the lens element 60, and diverging from
the central beam axis 70 (which passes through the telecentric
lens' focal point 62) is definable in accordance with the
two-dimensional spatial displacement from the beam axis 70 of its
associated emitter within the array 30. Thus, as shown in FIG. 4, a
beam b.sub.i generated by a laser emitter within the array 30 that
is relatively close to the axis 70 will undergo a smaller angle of
divergence through the focal point 62 from the axis 70, than will a
beam b.sub.j generated by a laser emitter that spaced farther away
from the axis.
This means that the desired travel path of an optical beam carrying
a particular signal channel may be readily defined by controlling
the crossbar switch 11 feeding the two-dimensional light beam
element array 30, so as to steer the signal from whichever one of
the switch's input ports 12 to which it is applied, to that one of
the switch output ports 13 whose associated light beam element in
the light element array 30 produces the intended travel path--based
upon the geometry parameters of the spatial separation-to-angular
divergence transform, described above.
Namely, the invention is able to project multiple transmit optical
signals from a two-dimensional planar array of optical emitters
into differentially divergent, free-space beams through a commonly
shared aperture of the telecentric lens, with a precise
relationship between the position of an emitter and it's angular
transmit direction. Conversely, in the receive or return path
direction, the telecentric lens accepts multiple receive optical
beams and directs them onto a two-dimensional receiver array. The
optics of the lens system produce a typical Fourier transform
operation, and the focal plane positions correspond to unique
angular beam directions.
The received beams at a prescribed optical receiver wavelength
.lambda..sub.R, are preferably defocused, so that their spots on an
opto-electronic receiver array 130 are appropriately larger than
any dead spaces of the array. This defocusing obviates the
requirement for diffraction-limited optical performance, so that
lens components 90 and 60 may be implemented as a pair of simple
spherical lenses.
As pointed out briefly above, for the point-to-multipoint
transceiver application of the present example, the dichroic
material-coated interface 55 of lens element 50 reflects light
received by face 52 from the lens element 60 toward the side face
56, to which an opto-electronic receiver array 130 is coupled. As
in the case of the transmitter array 30, although the light
receiver array 130 may comprise a linear (1.times.J) array of
hotodetector elements, it is preferably configured as a
two-dimensional array of photodetector elements, having a
sensitivity characteristic at optical receiver wavelength
.lambda..sub.R, different from the optical transmission wavelength
.lambda..sub.T.
As a non-limiting example, the photodetector array 130 may comprise
a photodetector array from Sensors Unlimited Inc., Princeton N.J.,
having a substantially planar input surface 132, to facilitate
intimately optically coupling the array with the substantially
planar side surface 56 of the lens element 50. Where the
transceiver application provides duplex communications with each
remote site, the photodetector array 130 may have effectively the
same size as the laser emitter array 30, so that its photodetector
elements are readily aligned with the input beams directed thereon
from the remote sites by the telecentric lens.
The photodetector array 130 has its signal output ports connected
to respective signal inputs of a set of receiver demodulators 140,
outputs of which are coupled to signal inputs of an X input by Y
output crossbar switch 150. The output crossbar switch 150 may be
configured complementary to the input crossbar switch 11, so that
X=N and Y=M. As such, the output signals from the output crossbar
switch 150 may be supplied to digital subscriber lines coupled to
the transmit side crossbar switch 11 for the case of duplex
communications. In a complementary sense to the transmit crossbar
switch 11, the receiver crossbar switch 150 serves to enable a
signal applied to any of X=N input ports 151 from the receiver
demodulator circuitry 140 to be controllably electronically steered
to any of its Y=M output ports.
Also shown in FIG. 4 is an auxiliary tracking (two-dimensional)
photodetector array 160 coupled with an associated focusing lens
162. Array 160 may comprise a conventional charge-coupled device
(CCD) receiver array. The outputs of the tracking array 160 are
coupled to the control processor 100, which defines the spatial
steering of the signal beams through its control of the crossbar
switches 30 and 130, as described above. The auxiliary array 160 is
used to monitor one or more beams from nodes whose spatial
locations relative to the hub site are precisely known a priori.
Any offset in the spatial location of a `tracking` beam from such a
node on the tracking array 160 is used as an error correction
signal by the control processor to impart the appropriate (X-Y)
correction, as needed, in the steering commands supplied by the
control processor 100 to the crossbar switches 30 and 130, so as to
provide for real-time pointing/tracking and atmospheric correction
capability.
While the optical transceiver embodiment shown in FIGS. 4 and 5 may
employ conventional spherical lenses, as described above, it should
be realized that there may be significant cost and performance
advantages in using other components, such as diffractive optical
elements (DOEs) or holographic optical elements (HOEs), it being
understood that the wavelength-dependent aspects of such elements
must be taken into account in the course of configuring a
two-wavelength transceiver system.
Also, although the AMOX architecture described above allows all of
the transmit and receive beams to share a common aperture, this is
not a functional necessity. In certain applications, it may be
advantageous to split the transmit and receive beams between two
spatially separate apertures, so that (transmit vs. receive)
wavelength segregation employed in the embodiment of FIGS. 4 and 5
is not necessary. Potential advantages of such beam division
include larger receiver apertures for improved signal collection,
optimization to specific transmit and receive array configurations,
and a reduction in the complexity of DOE/HOE's optical elements
(where applicable).
As shown in FIG. 6, an optical configuration for such a
unidirectional terminal is similar to that shown in FIG. 5, except
for the absence of a dichroic beam splitter, for a respective
transmit or receive portion of an AMOX architecture. Here, the
terminal serves as an adaptive multi-beam optical transmitter
(AMOT) or an adaptive multi-beam optical receiver (AMOR). Whether
implementing an AMOX, AMOT, or AMOR, the components of the optical
system can be readily scaled to specific arrays and beam-forming
requirements. A significant amount of flexibility is therefore
available to accommodate a wide range of system applications
including interdependent variations in field-of-regard (FOR), data
rates, link ranges, etc.
Regardless of whether an integrated transit/receive embodiment or a
segregated transmit and receive embodiment is employed, the beams
share a common aperture, so that there is a contiguous near-field
beam coverage over the full FOR. In addition, as shown in the
beam-forming geometry diagram of FIG. 7, where the angle
.THETA..sub.i between adjacent transmitted beams is no more than
the angular beam width .THETA..sub.d between (for example -3 dB
beam edges), there will be a contiguous beam coverage in the
far-field as well. The most efficient use of beam space occurs with
.THETA..sub.i =.THETA..sub.d. In this case, neighboring beams
become "resolvable" (e.g., centerlines are separated by one-half a
beamwidth) at a distance of L.sub.s =D.sub.t /tan
.THETA..sub.i.
Transmitter arrays for the above-described FSOC terminal may be
implemented in a variety of ways. In a relatively simple
application having only a small number of remote nodes, which are
also generally spatially stable, sub-populated non-switchable
arrays may be employed. A principal example of such a `non-agile`
application involves the use of an Ethernet network to `locally`
connect buildings that are in reasonably close proximity to one
another.
For such an application, a relatively limited set of discrete laser
sources 191 are coupled to receive electrical input signals in the
embodiment of FIG. 8 and optical input signals 196 to laser
amplifiers 195 in the embodiment of FIG. 9. The outputs of the
lasers may be hard-wired via an array of associated optical fibers
192 to respective spatial locations 193 within a light emitter
array plane 194, for which the spatial-to-angular transform
produced by the telecentric lens will direct the emitter beams
along the desired angular directions of the subscriber nodes. A
benefit of the fiber optic input embodiment of FIG. 9 is the fact
that a respective input signal may require only optical
amplification prior to being transmitting into free space.
Optionally, the fiber optic array may comprise a fully populated
fiber bundle, in which only specific fibers are connected to laser
sources based on subscriber demand. In either case, M input data
channels are specifically mapped to M output beam directions, as
shown.
Although the invention may be applied to such `non-agile`
multi-beam terminals, the preferred embodiment of the invention
employs the `agile` configuration described above with reference to
FIGS. 4-7, in which any array position is potentially active and
dynamically addressable. A non-limiting application of an agile
array would be to allow mobile communication personnel to rapidly
deploy a local area network (LAN), while providing for dynamic
variations in the number and/or physical locations of the nodes of
the network, and to track and correct for relative motion between
the nodes.
In order to realize cost-effective, agile transmitter arrays,
vertical cavity surface-emitting laser (VCSEL) components may be as
the array 30 in combination with an M.times.N digital crossbar
switch, as diagrammatically illustrated in the architecture of FIG.
10. Alternatively, as shown in the embodiment of FIG. 11, the
VCSELs may be replaced by discrete laser diodes 191 in a
sub-populated array, similar to the embodiment of FIG. 8. An
advantage of using VCSELs is their ability to simultaneously emit
multiple transverse modes. For reasons similar to the ability of a
light-emitting diode (LED) to eliminate modal noise in a multi-mode
fiber link, a multi-transverse mode (MTM) source may also
significantly reduce the effects of atmospheric scintillation in a
FSOC link.
Scintillation is the result of multi-path propagation in the
atmosphere due to inhomogeneities in the index of refraction of
air, causing the beam to temporally interfere with itself, both
constructively and destructively. With an MTM source, however, the
beam has already been somewhat "pre-scrambled" or homogenized, so
the effects of additional phase scrambling due to scintillation may
be greatly reduced, in comparison with problems that can occur with
a single-transverse-mode source. This effect may also be generated
or enhanced by using a custom-designed optical element to scramble
the phase-fronts prior to transmission. As a non-limiting example,
a DOE/HOE or a simple diffuser may be employed. This technique may
also be used to produce the desired beam angle for the intended
application, thereby efficiently accomplishing both objectives.
The M.times.N digital crossbar switches described above may be
implemented in a variety of ways, such as, but not limited to
application specific integrated circuits (ASICs), and logically
controlled high-speed switches (LCHSSs). An ASIC implementation has
several significant technical advantages, including very high
packaging density (only one chip), reliability, and lower power
requirements. However, in small quantities, ASICs may not be
practical, due to their high set-up costs and the long continuing
backlog at ASIC foundries. The LCHSS approach interconnects several
high-speed digital switches to route the data signals and a field
programmable gate array (FPGA), to control the configuration of the
switches. This implementation is relatively low cost and can be
packaged in a small volume.
The electrical bias of the laser emitters of the transmit array
must also be individually controlled to maintain overall low power
operation and to reduce the effects of heat buildup. For example,
if a maximum of ten simultaneous transmit beams is employed, the
emitters can be controlled with ten current sources, that are
switched to the lasers via semiconductor switches and controlled by
the same FPGA used to control the data switches.
Alternatively, the crossbar switch may be implemented as an
all-optical fiber optic switch, as diagrammatically illustrated at
120 in FIGS. 12 and 13. A principal advantage of an optical fiber
approach is that the number of laser elements 191 (e.g., lasers
having a transmit wavelength of 1550 nm) can be reduced to match
the number of input signals. In the embodiments of FIGS. 12 and 13,
a transmit element array 123 is formed of a fully populated fiber
optic bundle, which can be configured and sized to have the desired
element center-to-center spacing.
Consistent with point-to-multipoint (PMP) applications, preliminary
link analyses have been performed to explore inter-related issues
of data rate, link range, beam width, number of array elements,
optical power, addressable field-of-regard (FOR), background
optical noise, etc. The subscriber nodes in the PMP network are
assumed to be single-channel (i.e., single laser, single detector).
As such, they may employ collection apertures and transmit beam
widths consistent with closing a duplex link with the multi-channel
hub terminal in a conventional manner. As a non-limiting example, a
fixed subscriber collection aperture of 6.0 inches may be
assumed.
FIG. 14 shows an example of compiling the results of many link
analyses to determine array size (number of required emitters) and
addressable field-of-regard (FOR). In particular, FIG. 14
illustrates the number of array emitters required to cover FOR's
ranging between 30.degree. and 90.degree. at data rates of 39 and
622 Mbps. For instance, to operate at a range of 1 km, a data rate
of 622 Mbps, and a FOR of 30.degree..times.90.degree., the transmit
array requires on the order of 20.times.60 emitter elements.
The receiver array generally requires a more complicated
implementation than the transmitter array. Although, as described
above, the transmitter array may comprise a spatially periodic,
two-dimensional array of point-source emitters, the beams impinging
upon the receiver array can be expected to be incident at arbitrary
locations within the array depending on the angular position of
subscriber nodes. The receiver array elements should therefore have
the largest possible active area (up to the desired spatial
resolution of the array) and the highest possible fill-factor (or
very little dead space between photodetector elements).
In addition, the node connecting the detector, preamplifier, and
feedback resistor components of a respective photodetector element
must be relatively `physically short` in order to preserve the
receiver's bandwidth performance. In a two-dimensional receiver
array, this node length may become unacceptable due to the loss of
the second dimension for mounting components. To obviate this
problem the detector's preamplifier may be co-mounted on the
detector substrate. Alternatively, the receiver array may be
configured as diagrammatically illustrated in FIG. 15, which shows
the collection of the received beams on the end of a fiber bundle
125, the outputs of which are (optical-fiber) routed via a set of
fiber optic switches 120 to a subset of optimized discrete
photodetectors 127.
This receiver architecture of FIG. 15 is essentially the inverse of
the transmit array architecture of FIG. 12, described above. In
order to achieve a high fill-factor, a respective optical fiber may
contain a multimode core with a relatively thin cladding layer,
such as a 100/125 micron core/cladding diameter. A 100 micron core
provides a relatively good match to the active area of a
high-performance photodetector operating in excess of 1 Gbps. The
fibers from the bundle 221 can be physically `fanned`, as
necessary, in order to interface with the fiber optic switch 223.
High-density packaging of the receiver modules 223 can be enhanced
by using integrated receiver arrays, which are currently
commercially available in packages of up to 1.times.16 on a single
substrate.
As will be appreciated from the foregoing description, the present
invention takes advantage of current and emerging technology
developments in free-space optical communications, to realize an
electronically agile multi-beam optical transceiver for use in a
point-to-multipoint hub. This agile transceiver allows any of
multiple optical beams to be dynamically routed and spatially
re-directed, in respectively different directions through a common
optical aperture over a relatively wide field to a plurality of
spatially diverse sites or nodes. In addition, a tracking array
actively corrects for pointing and tracking errors that may be due
to relative node motions and atmospheric induced distortions.
Having no moving parts, the invention provides a reduction in size,
weight, and cost, while improving reliability and
functionality.
While we have shown and described several embodiments in accordance
with the present invention, it is to be understood that the same is
not limited thereto but is susceptible to numerous changes and
modifications as known to a person skilled in the art. We therefore
do not wish to be limited to the details shown and described
herein, but intend to cover all such changes and modifications as
are obvious to one of ordinary skill in the art.
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